The present invention relates generally to a device for analyzing a small sample in a test chamber using infrared (IR) spectrophotometry. More specifically, the invention relates to a device and a method for detecting unwanted formations in liquid samples wherein the system maintains a dry atmosphere around the sample, and the sample is cooled to a desired temperature before and during scans.
Scientists have discovered that chlorofluorocarbons (CFC's) deplete ozone in the stratosphere. Ozone is the substance which protects us from the harmful effects of ultraviolet radiation. As a result, many governments have made a mandate to cease production of CFC's.
One of the most common refrigerants used today is CFC-12. Refrigeration system manufacturers throughout the industry have found HFC-134a to be a suitable alternative to CFC-12. In the initial phases of testing, it was soon discovered that certain contaminants in HFC-134a refrigeration systems would cause failures. Certain compounds are not soluble in HFC-134a at the low temperatures present in a refrigeration system. These substances tend to solidify in the small diameter of the expansion device. The reduced flow of refrigerant through the system results in a loss of the cooling effect. Only a few milligrams of deposit can cause a failure. The most common substance which is known to cause this failure is paraffin. It is a common component of many lubricants. These lubricants are used in the manufacture of refrigeration system components. Other hydrocarbons are also commonly used in these lubricants.
To eliminate paraffin from the refrigeration system, an analytical technique is needed to identify the paraffin in small samples. Paraffin is a straight chain hydrocarbon. Depending on its molecular weight, it can be solid liquid or even gaseous at room temperature. Common lubricants contain a mixture of solid and liquid paraffins. The solid paraffins are dissolved in the liquid paraffins resulting in a liquid product. When cooled, these paraffins will solidify. At a temperature of -30.degree. F., other hydrocarbons, such as naphthenic or polyalphaolefins, do not solidify. This is why they cause plugging in HFC-134a refrigeration systems. The IR spectrum of all three hydrocarbons is similar except for paraffin. A transition occurs in the IR spectrum when paraffins go from a liquid to a solid state. This is shown in FIG. 4. As illustrated, a singlet band at around 722 cm.sup.-1 splits to form a doublet band. The doublet band generally occurs at 718 cm.sup.-1 and 728 cm.sup.-1. Thus, by cooling the sample and analyzing it by IR spectrophotometry, paraffin can be identified. Other analytical techniques are available; however, they require expensive equipment and lengthy test times.
One such known apparatus for analyzing a sample cell using infrared spectrophotometry is disclosed in U.S. Pat. No. 5,280,177. In the '177 patent, cooling of a sample to temperatures as low as -45.degree. C. is taught. To this end, a cool air passageway is provided adjacent a primary optical surface of a sample holder for directing a cool air stream across the primary optical surface. A vortex tube has a cool air outlet connected to the cool air passageway for supplying cool air to the passageway. The system disclosed in the '177 patent, however, is difficult to maintain very cool temperatures, and attaining of extreme temperature is often difficult within a short period of time. Further, moisture causing condensation contamination on a test disk and outside the chamber disk is a danger to the system described in the '177 patent.
A need, therefore, exists for an improved system and a method for analyzing a sample in a test chamber using infrared spectrophotometry.